The present invention relates to a device for restricting the differential operation of a differential of a motor vehicle, and more particularly to a device comprising a fluid-operated multiple-disk friction clutch for controlling restriction torque in accordance with driving conditions of the motor vehicle.
In a motor vehicle, a differential is provided between the right and left driving wheels to absorb the difference in speeds of the wheels during turning of a corner and distribute driving torque equally to the driving wheels. However, when one of the wheels slips due to a low friction coefficient of the road surface or because of a decrease in the load on the wheel, the total distributed driving torque decreases. In order to prevent such a decrease of the torque, a limited slip differential (LSD) where a multiple-disk friction clutch is disposed therein is provided to automatically restrict the differential operation, thereby transmitting sufficient torque to the wheels to drive the vehicle. However, the LSD is operated to restrict the differential operation in accordance with the change in the driving torque, namely with the operation of the accelerator pedal of the vehicle. Therefore, if the accelerator pedal is depressed or released while turning a corner, it becomes difficult to steer the vehicle because of the operation of the LSD. The LSD is therefore used only for a rear differential of a rear drive vehicle, or of a four-wheel drive vehicle, or for a front differential in a particular type of a front drive vehicle.
A viscous coupling containing a high viscous fluid recently has been proposed for restricting the differential operation of the differential. The viscous coupling operates to generate a coupling torque in proportion to the difference between right-wheel speed and left-wheel speed. Thus, the vehicle can be steered while the differential operation is restricted despite the operation of the accelerator pedal.
However, since the coupling torque depends only on the difference between the speeds of the right and left wheels, it is also necessary to determine the torque capacity of the viscous coupling in consideration of the difference in effective diameter of the wheels and of deterioration of the coupling. Particularly, in a vehicle having an antilock braking system (ABS) on a brake system, a viscous coupling having a small torque capacity is preferable to prevent locking thereof when the antilock braking is effected. Hence the differential operation may not always be restricted when necessary. In addition, the viscous coupling is operated only in response to the speed difference, and cannot be intentionally controlled in accordance with the driving conditions and conditions of the road surface.
Japanese Patent Application Laid-Open No. 62-103227 discloses a system for controlling the differential operation of a differential disposed between right and left wheels. In the system, a fluid operated multiple-disk friction clutch is provided between a differential case and a side gear of a differential for producing the torque for restricting the differential operation. Actuating pressure of oil for the clutch is controlled for changing the differential operation restricting torque in dependency on the difference between the speeds of right and left wheels so as to improve the driveability of the motor vehicle. However, the structure of the clutch is too complicated to be easily installed in an ordinary differential.
In addition, the multiple-disk friction clutch uses the oil in the automatic transmission as actuating oil for both actuating and lubricating the clutch, since the oil has a good friction characteristic and a low oil viscosity. On the other hand, a final reduction gear of the differential provided on the outside of the differential case is composed of a hypoid gear. In such a differential, extreme-pressure lubricant contains additives to improve ability to adhere to the surfaces of metals under high bearing pressures. Such a lubricant may cause the multiple disks of the clutch to stick and to slip, which causes unpleasant vibration and noise of the vehicle body.
Moreover, the clutch is applied to an independently disposed rear differential of a rear drive vehicle with front-mounted engine or a four-wheel drive vehicle and not to a differential of a front drive vehicle with a front-mounted engine or a rear drive vehicle with a rear-mounted engine having a transmission of the transaxle type.